The invention relates to an amplifier circuit arrangement, a phase lock loop including such an amplifier arrangement, and a radio receiver including such an amplifier or phase lock loop.
When designing portable radio receivers, particularly narrow band multi-channel receivers, it is useful to employ automatic frequency control (AFC). This gives the advantage of maintaining the intermediate frequency (IF) signal in the center of the IF filter pass band when the local oscillator frequency drifts due to temperature changes or component ageing. Also the distortion produced in the receiver is minimized and matching of the crystal filter becomes less critical. In addition, the performance of a muting circuit can be maintained when the local oscillator frequency drifts.
However, in multi-channel receivers, the automatic frequency control circuit may be affected by strong signals in channels adjacent to the selected channel. This particularly applies in a receiver which works on the principle of a dual phase-locked loop. Such a receiver is described in a paper read by Stephen W. Watkinson at the Communications 74 Conference held in Brighton, England, during July 1974 and published in the Conference Proceedings at pages 13.1/1 to 13.1/8, the contents of which are hereby incorporated by reference. In this system a very high level adjacent channel signal will produce a beat frequency with the intermediate frequency voltage controlled crystal oscillator (IFVCXO). This waveform will frequency modulate the IFVCXO producing a non-sinusoidal error waveform containing a d.c. component. The d.c. component will be fed to the local oscillator causing its frequency to be pulled towards the frequency of the adjacent channel signal thus reducing the rejection of the adjacent channel signal. If the level of the adjacent signal is now reduced, the pulling effect may be maintained typically down to a level to 30 dB below that at which is started.
This problem may be reduced by including a low-pass filter in the feed-back path to the IFVCXO but if the filter has a rapid roll-off, the loop may become unstable. In practice, the loop filter must be limited to a single pole network and consequently, for a 12.5 KHz channel spacing, the attenuation of the 12.5 KHz beat frequency is not sufficient to overcome the problem. An alternative arrangement would be to include a notch filter such as a Twin-Tee circuit, with the notch frequency at the difference between the selected channel and adjacent channel frequencies. However, if the notch is too deep, loop instability will again be produced because of the reactive components in the notch filter circuit.